Apparatus and method of transmitting and receiving information in a wireless telecommunications system

ABSTRACT

A wireless telecommunications system (1) includes a central terminal (10) for transmitting and receiving radio frequency signals to and from a subscriber terminal (20). A downlink communication path is established from a transmitter (200) of the central terminal (10) to a receiver (202) of the subscriber terminal (20). A downlink signal (212) is transmitted from the transmitter (200) to the receiver (202) during setup and operation of the wireless telecommunications system (1). The wireless telecommunications system (1) operates in one of three operating modes. In an acquisition mode during establishment of the downlink communication path, the downlink signal (212) is transmitted at a high power level and a low transmit rate with the receiver (202) operating at the low transmit rate. In a standby mode after establishment of the downlink communication path, the downlink signal (212) is transmitted at a low power level and a low transmit rate with the receiver (202) operating at the low transmit rate. In a traffic mode upon a request for wireless communication transmission, the downlink signal (212) is transmitted at a high power level and a high transmit rate with the receiver (202) adjusting to operate at the high transmit rate. Upon completion of the wireless communication transmission, the wireless telecommunications system (1) returns to the standby mode and the receiver (202) adjusts to operate at the low transmit rate.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to telecommunications systemsand more particularly to an apparatus and method of transmitting andreceiving information in a wireless telecommunications system.

BACKGROUND OF THE INVENTION

Wireless telecommunications system use transmitters and receivers tosend and receive radio frequency signals in a telephone-like networkconfiguration. The wireless telecommunications systems require radiolinks be established between corresponding transmitters and receivers inorder to provide wireless communication transmissions. Interference anddelays occur in achieving acquisition and establishment of each radiolink. It becomes inefficient to acquire and establish a radio link everytime a call is to be initiated. Problems with interference also occur ifa radio link is maintained whether or not a call is in progress due topower needed to maintain the radio link. Therefore, it is desirable toavoid establishing a radio link for each call and avoid insertinginterference into the wireless telecommunications system throughcontinuous maintenance of a radio link.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus and/ormethod of transmitting and receiving information in a wirelesstelecommunications system that substantially eliminates or reducesdisadvantages and problems associated with conventional wirelesscommunication techniques.

In accordance with one aspect of the present invention, there isprovided a method of transmitting information in a wirelesstelecommunications system, comprising the steps of:

transmitting a downlink signal at a first transmitting power and a firsttransmitting rate in an acquisition mode;

transmitting the downlink signal at a second transmitting power and thefirst transmitting rate in a standby mode upon completion of theacquisition mode.

In accordance with another aspect of the present invention, there isprovided a system for transmitting and receiving information in awireless telecommunications system, comprising:

a transmitter in a central terminal operable to transmit a downlinksignal, the downlink signal transmitted at a first transmitting powerand a first transmitting rate in an acquisition mode duringestablishment of a downlink communication path;

a receiver in a subscriber terminal operable to receive the downlinksignal in the acquisition mode at the first transmitting rate, thereceiver establishing the downlink communication from the transmitter inthe central terminal to the receiver in the subscriber terminal, thetransmitter transmitting the downlink signal at a second transmittingpower and the first transmitting rate in a standby mode in response toestablishment of the downlink communication path, the receiver operableto receive the downlink signal in the standby mode at the firsttransmitting rate.

In accordance with a further aspect of the present invention, there isprovided subscriber terminal in a wireless telecommunications system,comprising:

a receiver operable to receive a downlink signal in an acquisition modeto establish a downlink communication path to the receiver, the downlinksignal having a first transmitting power and a first transmitting ratein the acquisition mode, the receiver operating at the firsttransmitting rate in the acquisition mode to receive the downlinksignal, the downlink signal having a second transmitting power and thefirst transmitting rate in a standby mode upon establishing the downlinkcommunication path, the receiver operating at the first transmittingrate in the standby mode.

According to an embodiment of the present invention, a method oftransmitting information in a wireless telecommunications systemincludes transmitting a downlink signal at a first transmitting powerand first transmitting rate in an acquisition mode to establish adownlink communication path. The downlink signal is transmitted at asecond transmitting power and the first transmitting rate in a standbymode after establishment of the downlink communication path. Thedownlink signal is transmitted at the first transmitting power and asecond transmitting rate upon a request for a wireless telephone call.

The present invention provides various technical advantages overconventional wireless communication techniques. For example, onetechnical advantage is to provide multiple operating modes havingdifferent transmitting powers and different transmitting rates. Anothertechnical advantage is to provide low transmitting power at a lowtransmit rate during system idle periods. Yet another technicaladvantage is to efficiently change between different transmitting powersand different transmit rates. Other technical advantages are readilyapparent to one skilled in the art from the following figures,description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will be described hereinafter, by way ofexample only, with reference to the accompanying drawings in which likereference signs are used for like features and in which:

FIG. 1 is a schematic overview of an example of a wirelesstelecommunications system in which an example of the present inventionis included;

FIG. 2 is a schematic illustration of an example of a subscriberterminal of the telecommunications system of FIG. 1;

FIG. 3 is a schematic illustration of an example of a central terminalof the telecommunications system of FIG. 1;

FIG. 3A is a schematic illustration of a modem shelf of a centralterminal of the telecommunications system of FIG. 1;

FIG. 4 is an illustration of an example of a frequency plan for thetelecommunications system of FIG. 1;

FIGS. 5A and 5B are schematic diagrams illustrating possibleconfigurations for cells for the telecommunications system of FIG. 1;

FIG. 6 is a schematic diagram illustrating aspects of a code divisionmultiplex system for the telecommunications system of FIG. 1;

FIG. 7 is a schematic diagram illustrating signal transmissionprocessing stages for the telecommunications system of FIG. 1;

FIG. 8 is a schematic diagram illustrating signal reception processingstages for the telecommunications system of FIG. 1;

FIG. 9 is a schematic diagram illustrating downlink and uplinkcommunication paths for the wireless telecommunications system;

FIG. 10 is a schematic diagram illustrating the makeup of a downlinksignal transmitted by the central terminal;

FIG. 11 is a graphical depiction illustrating the phase adjustment to aslave code sequence of the subscriber terminal;

FIG. 12 is a graphical depiction of a signal quality estimate performedby the receiver in the subscriber terminal;

FIG. 13 is a graphical diagram illustrating the contents of a frameinformation signal within the downlink signal;

FIG. 14 is a tabular depiction illustrating overhead insertion into adata stream of the downlink signal;

FIG. 15 is a tabular depiction of a power control signal in an overheadchannel of the downlink signal;

FIG. 16 is a tabular depiction of a code synchronization signal in theoverhead channel of the downlink signal;

FIG. 17 is a graphical depiction of a transmitting power and a transmitrate for each mode of operation of the wireless telecommunicationssystem; and

FIG. 18 is a schematic diagram illustrating the operation of thereceiver and transmitter in the subscriber terminal.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic overview of an example of a wirelesstelecommunications system. The telecommunications system includes one ormore service areas 12, 14 and 16, each of which is served by arespective central terminal (CT) 10 which establishes a radio link withsubscriber terminals (ST) 20 within the area concerned. The area whichis covered by a central terminal 10 can vary. For example, in a ruralarea with a low density of subscribers, a service area 12 could cover anarea with a radius of 15-20 Km. A service area 14 in an urbanenvironment where is there is a high density of subscriber terminals 20might only cover an area with a radius of the order of 100 m. In asuburban area with an intermediate density of subscriber terminals, aservice area 16 might cover an area with a radius of the order of 1 Km.It will be appreciated that the area covered by a particular centralterminal 10 can be chosen to suit the local requirements of expected oractual subscriber density, local geographic considerations, etc, and isnot limited to the examples illustrated in FIG. 1. Moreover, thecoverage need not be, and typically will not be circular in extent dueto antenna design considerations, geographical factors, buildings and soon, which will affect the distribution of transmitted signals.

The central terminals 10 for respective service areas 12, 14, 16 can beconnected to each other by means of links 13, 15 and 17 which interface,for example, with a public switched telephone network (PSTN) 18. Thelinks can include conventional telecommunications technology usingcopper wires, optical fibers, satellites, microwaves, etc.

The wireless telecommunications system of FIG. 1 is based on providingfixed microwave links between subscriber terminals 20 at fixed locationswithin a service area (e.g., 12, 14, 16) and the central terminal 10 forthat service area. In a preferred embodiment each subscriber terminal 20is provided with a permanent fixed access link to its central terminal10. However, in alternative embodiments demand-based access could beprovided, so that the number of subscribers which can be servicedexceeds the number of telecommunications links which can currently beactive.

FIG. 2 illustrates an example of a configuration for a subscriberterminal 20 for the telecommunications system of FIG. 1. FIG. 2 includesa schematic representation of customer premises 22. A customer radiounit (CRU) 24 is mounted on the customer's premises. The customer radiounit 24 includes a flat panel antenna or the like 23. The customer radiounit is mounted at a location on the customer's premises, or on a mast,etc., and in an orientation such that the flat panel antenna 23 withinthe customer radio unit 24 faces in the direction 26 of the centralterminal 10 for the service area in which the customer radio unit 24 islocated.

The customer radio unit 24 is connected via a drop line 28 to a powersupply unit (PSU) 30 within the customer's premises. The power supplyunit 30 is connected to the local power supply for providing power tothe customer radio unit 24 and a network terminal unit (NTU) 32. Thecustomer radio unit 24 is also connected to via the power supply unit 30to the network terminal unit 32, which in turn is connected totelecommunications equipment in the customer's premises, for example toone or more telephones 34, facsimile machines 36 and computers 38. Thetelecommunications equipment is represented as being within a singlecustomer's premises. However, this need not be the case, as thesubscriber terminal 20 preferably supports either a single or a dualline, so that two subscriber lines could be supported by a singlesubscriber terminal 20. The subscriber terminal 20 can also be arrangedto support analogue and digital telecommunications, for example analoguecommunications at 16, 32 or 64 kbits/sec or digital communications inaccordance with the ISDN BRA standard.

FIG. 3 is a schematic illustration of an example of a central terminalof the telecommunications system of FIG. 1. The common equipment rack 40comprises a number of equipment shelves 42, 44, 46, including a RFCombiner and power amp shelf (RFC) 42, a Power Supply shelf (PS) 44 anda number of (in this example four) Modem Shelves (MS) 46. The RFcombiner shelf 42 allows the four modem shelves 46 to operate inparallel. It combines and amplifies the power of four transmit signals,each from a respective one of the four modem shelves, and amplifies andsplits received signals four way so that separate signals may be passedto the respective modem shelves. The power supply shelf 44 provides aconnection to the local power supply and fusing for the variouscomponents in the common equipment rack 40. A bidirectional connectionextends between the RF combiner shelf 42 and the main central terminalantenna 52, typically an omnidirectional antenna, mounted on a centralterminal mast 50.

This example of a central terminal 10 is connected via a point-to-pointmicrowave link to a location where an interface to the public switchedtelephone network 18, shown schematically in FIG. 1, is made. Asmentioned above, other types of connections (e.g., copper wires oroptical fibers) can be used to link the central terminal 10 to thepublic switched telephone network 18. In this example the modem shelvesare connected via lines 47 to a microwave terminal (MT) 48. A microwavelink 49 extends from the microwave terminal 48 to a point-to-pointmicrowave antenna 54 mounted on the mast 50 for a host connection to thepublic switched telephone network 18.

A personal computer, workstation or the like can be provided as a sitecontroller (SC) 56 for supporting the central terminal 10. The sitecontroller 56 can be connected to each modem shelf of the centralterminal 10 via, for example, RS232 connections 55. The site controller56 can then provide support functions such as the localization offaults, alarms and status and the configuring of the central terminal10. A site controller 56 will typically support a single centralterminal 10, although a plurality of site controllers 56 could benetworked for supporting a plurality of central terminals 10.

As an alternative to the RS232 connections 55, which extend to a sitecontroller 56, data connections such as an X.25 links 57 (shown withdashed lines in FIG. 3) could instead be provided from a pad 228 to aswitching node 60 of an element manager (EM) 58. An element manager 58can support a number of distributed central terminals 10 connected byrespective connections to the switching node 60. The element manager 58enables a potentially large number (e.g., up to, or more than 1000) ofcentral terminals 10 to be integrated into a management network. Theelement manager 58 is based around a powerful workstation 62 and caninclude a number of computer terminals 64 for network engineers andcontrol personnel.

FIG. 3A illustrates various parts of a modem shelf 46. Atransmit/receive RF unit (RFU--for example implemented on a card in themodem shelf) 66 generates the modulated transmit RF signals at mediumpower levels and recovers and amplifies the baseband RF signals for thesubscriber terminals. The RF unit 66 is connected to an analogue card(AN) 68 which performs A-D/D-A conversions, baseband filtering and thevector summation of 15 transmitted signals from the modem cards (MCs)70. The analogue unit 68 is connected to a number of (typically 1-8)modem cards 70. The modem cards perform the baseband signal processingof the transmit and receive signals to/from the subscriber terminals 20.This includes 1/2 rate convolution coding and x 16 spreading with CDMAcodes on the transmit signals, and synchronization recovery, despreadingand error correction on the receive signals. Each modem card 70 in thepresent example has two modems, each modem supporting one subscriberlink (or two lines) to a subscriber terminal 20. Thus, with two modemsper card and 8 modems per modem shelf, each modem shelf could support 16possible subscriber links. However, in order to incorporate redundancyso that a modem may be substituted in a subscriber link when a faultoccurs, only up to 15 subscriber links are preferably supported by asingle modem shelf 46. The 16th modem is then used as a spare which canbe switched in if a failure of one of the other 15 modems occurs. Themodem cards 70 are connected to the tributary unit (TU) 74 whichterminates the connection to the host public switched telephone network18 (e.g., via one of the lines 47) and handles the signaling oftelephony information to, for example, up to 15 subscriber terminals(each via a respective one of 15 of the 16 modems).

The wireless telecommunications between a central terminal 10 and thesubscriber terminals 20 could operate on various frequencies. FIG. 4illustrates one possible example of the frequencies which could be used.In the present example, the wireless telecommunication system isintended to operate in the 1.5-2.5 GHz Band. In particular the presentexample is intended to operate in the Band defined by ITU-R (CCIR)Recommendation F.701 (2025-2110 MHz, 2200-2290 MHz). FIG. 4 illustratesthe frequencies used for the uplink from the subscriber terminals 20 tothe central terminal 10 and for the downlink from the central terminal10 to the subscriber terminals 20. It will be noted that 12 uplink and12 downlink radio channels of 3.5 MHz each are provided centered about2155 MHz. The spacing between the receive and transmit channels exceedsthe required minimum spacing of 70 MHz.

In the present example, as mentioned above, each modem shelf willsupport 1 frequency channel (i.e. one uplink frequency plus thecorresponding downlink frequency). Up to 15 subscriber links may besupported on one frequency channel, as will be explained later. Thus, inthe present embodiment, each central terminal 10 can support 60 links,or 120 lines.

Typically, the radio traffic from a particular central terminal 10 willextend into the area covered by a neighboring central terminal 10. Toavoid, or at least to reduce interference problems caused by adjoiningareas, only a limited number of the available frequencies will be usedby any given central terminal 10.

FIG. 5A illustrates one cellular type arrangement of the frequencies tomitigate interference problems between adjacent central terminals 10. Inthe arrangement illustrated in FIG. 5A, the hatch lines for the cells 76illustrate a frequency set (FS) for the cells. By selecting threefrequency sets (e.g., where: FS1=F1, F4, F7, F10; FS2=F2, F5, F8, F11;FS3=F3, F6, F9, F12), and arranging that immediately adjacent cells donot use the same frequency set (see, for example, the arrangement shownin FIG. 5A), it is possible to provide an array of fixed assignmentomnidirectional cells where interference between nearby cells can beavoided. The transmitter power of each central terminal 10 is set suchthat transmissions do not extend as far as the nearest cell which isusing the same frequency set. Thus each central terminal 10 can use thefour frequency pairs (for the uplink and downlink, respectively) withinits cell, each modem shelf in the central terminal 10 being associatedwith a respective RF channel (channel frequency pair).

With each modem shelf supporting one channel frequency (with 15subscriber links per channel frequency) and four modem shelves, eachcentral terminal 10 will support 60 subscriber links (i.e., 120 lines).The 10 cell arrangement in FIG. 5A can therefore support up to 600 ISDNlinks or 1200 analogue lines, for example. FIG. 5B illustrates acellular type arrangement employing sectored cells to mitigate problemsbetween adjacent central terminals 10. As with FIG. 5A, the differenttype of hatch lines in FIG. 5B illustrate different frequency sets. Asin FIG. 5A, FIG. 5B represents three frequency sets (e.g., where:FS1=F1, F4, F7, F10; FS2=F2, F5, F8, F11; FS3=F3, F6, F9, F12). However,in FIG. 5B the cells are sectored by using a sectored central terminal(SCT) 13 which includes three central terminals 10, one for each sectorS1, S2 and S3, with the transmissions for each of the three centralterminals 10 being directed to the appropriate sector among S1, S2 andS3. This enables the number of subscribers per cell to be increasedthree fold, while still providing permanent fixed access for eachsubscriber terminal 20.

A seven cell repeat pattern is used such that for a cell operating on agiven frequency, all six adjacent cells operating on the same frequencyare allowed unique PN codes. This prevents adjacent cells frominadvertently decoding data.

As mentioned above, each channel frequency can support 15 subscriberlinks. In this example, this is achieved using by multiplexing signalsusing a Code Division Multiplexed Access (CDMA) technique. FIG. 6 givesa schematic overview of CDMA encoding and decoding.

In order to encode a CDMA signal, base band signals, for example theuser signals for each respective subscriber link, are encoded at 80-80Ninto a 160 ksymbols/sec baseband signal where each symbol represents 2data bits (see, for example the signal represented at 81). This signalis then spread by a factor of 16 using a respective Walsh pseudo randomnoise (PN) code spreading function 82-82N to generate signals at aneffective chip rate of 2.56 Msymbols/sec in 3.5 MHz. The signals forrespective subscriber links are then combined and converted to radiofrequency (RF) to give multiple user channel signals (e.g., 85) fortransmission from the transmitting antenna 86.

During transmission, a transmitted signal will be subjected tointerference sources 88, including external interference 89 andinterference from other channels 90. Accordingly, by the time the CDMAsignal is received at the receiving antenna 91, the multiple userchannel signals may be distorted as is represented at 93.

In order to decode the signals for a given subscriber link from thereceived multiple user channel, a Walsh correlator 94-94N uses the samepseudo random noise (PN) code that was used for the encoding for eachsubscriber link to extract a signal (e.g, as represented at 95) for therespective received baseband signal 96-96N. It will be noted that thereceived signal will include some residual noise. However, unwantednoise can be removed using a low pass filter and signal processing.

The key to CDMA is the application of orthogonal codes that allow themultiple user signals to be transmitted and received on the samefrequency at the same time. Once the bit stream is orthogonally isolatedusing the Walsh codes, the signals for respective subscriber links donot interfere with each other.

Walsh codes are a mathematical set of sequences that have the functionof "orthonormality". In other words, if any Walsh code is multiplied byany other Walsh code, the results are zero.

FIG. 7 is a schematic diagram illustrating signal transmissionprocessing stages as configured in a subscriber terminal 20 in thetelecommunications system of FIG. 1. The central terminal is alsoconfigured to perform equivalent signal transmission processing. In FIG.7, an analogue signal from one of a pair of telephones is passed via atwo-wire interface 102 to a hybrid audio processing circuit 104 and thenvia a codec 106 to produce a digital signal into which an overheadchannel including control information is inserted at 108. The resultingsignal is processed by a convolutional encoder 110 before being passedto a spreader 116 to which the Rademacher-Walsh and PN codes are appliedby a RW code generator 112 and PN Code generator 114, respectively. Theresulting signals are passed via a digital to analogue converter 118.The digital to analogue converter 118 shapes the digital samples into ananalogue waveform and provides a stage of baseband power control. Thesignals are then passed to a low pass filter 120 to be modulated in amodulator 122. The modulated signal from the modulator 122 is mixed witha signal generated by a voltage controlled oscillator 126 which isresponsive to a synthesizer 160. The output of the mixer 128 is thenamplified in a low noise amplifier 130 before being passed via a bandpass filter 132. The output of the band pass filter 132 is furtheramplified in a further low noise amplifier 134, before being passed topower control circuitry 136. The output of the power control circuitryis further amplified in a further low noise amplifier 138 before beingpassed via a further band pass filter 140 and transmitted from thetransmission antenna 142.

FIG. 8 is a schematic diagram illustrating the equivalent signalreception processing stages as configured in a subscriber terminal 20 inthe telecommunications system of FIG. 1. The central terminal is alsoconfigured to perform equivalent signal reception processing. In FIG. 8,signals received at a receiving antenna 150 are passed via a band passfilter 152 before being amplified in a low noise amplifier 154. Theoutput of the amplifier 154 is then passed via a further band passfilter 156 before being further amplified by a further low noiseamplifier 158. The output of the amplifier 158 is then passed to a mixer164 where it is mixed with a signal generated by a voltage controlledoscillator 162 which is responsive to a synthesizer 160. The output ofthe mixer 164 is then passed via the de-modulator 166 and a low passfilter 168 before being passed to an analogue to digital converter 170.The digital output of the A/D converter 170 is then passed to acorrelator 178, to which the same Rademacher-Walsh and PN codes usedduring transmission are applied by a RW code generator 172(corresponding to the RW code generator 112) and a PN code generator 174(corresponding to PN code generator 114), respectively. The output ofthe correlator is applied to a Viterbi decoder 180. The output of theViterbi decoder 180 is then passed to an overhead extractor 182 forextracting the overhead channel information. The output of the overheadextractor 182 is then passed via a codec 184 and a hybrid circuit 188 toa two wire interface 190 where the resulting analogue signals are passedto a selected telephone 192.

At the subscriber terminal 20, a stage of automatic gain control isincorporated at the IF stage. The control signal is derived from thedigital portion of the CDMA receiver using the output of a signalquality estimator to be described later.

FIG. 9 is a block diagram of downlink and uplink communication pathsbetween central terminal 10 and subscriber terminal 20. A downlinkcommunication path is established from transmitter 200 in centralterminal 10 to receiver 202 in subscriber terminal 20. An uplinkcommunication path is established from transmitter 204 in subscriberterminal 20 to receiver 206 in central terminal 10. Once the downlinkand the uplink communication paths have been established in wirelesstelecommunication system 1, telephone communication may occur between afirst user 208 or a second user 210 of subscriber terminal 20 and a userserviced through central terminal 10 over a downlink signal 212 and anuplink signal 214. Downlink signal 212 is transmitted by transmitter 200of central terminal 10 and received by receiver 202 of subscriberterminal 20. Uplink signal 214 is transmitted by transmitter 204 ofsubscriber terminal 20 and received by receiver 206 of central terminal10. Downlink signal 212 and uplink signal 214 are transmitted as CDMAspread spectrum signals.

Receiver 206 and transmitter 200 within central terminal 10 aresynchronized to each other with respect to time and phase, and alignedas to information boundaries. In order to establish the downlinkcommunication path, receiver 202 in subscriber terminal 20 should besynchronized to transmitter 200 in central terminal 10. Synchronizationoccurs by performing an acquisition mode function and a tracking modefunction on downlink signal 212. Initially, transmitter 200 of centralterminal 10 transmits downlink signal 212. FIG. 10 shows the contents ofdownlink signal 212. Downlink signal 212 includes a code sequence signal216 for central terminal 10 combined with a frame information signal218. Code sequence signal 216 is derived from a combination of apseudo-random noise code signal 220 and a Rademacher-Walsh code signal222. Although FIG. 10 relates specifically to the makeup of the downlinksignal, the uplink has the same makeup.

Each receiver 202 of every subscriber terminal 20 serviced by a singlecentral terminal 10 operate off of the same pseudo-random noise codesignal as central terminal 10. Each modem shelf 46 in central terminal10 supports one radio frequency channel and fifteen subscriber terminals20, each subscriber terminal having a first user 208 and a second user210. Each modem shelf 46 selects one of sixteen Rademacher-Walsh codesignals 222, each Rademacher-Walsh code signal 222 corresponding to aunique subscriber terminal 20. Thus, a specific subscriber terminal 20will have an identical code sequence signal 218 as downlink signal 212transmitted by central terminal 10 and destined for the specificsubscriber terminal 20.

Downlink signal 212 is received at receiver 202 of subscriber terminal20. Receiver 202 compares its phase and code sequence to a phase andcode sequence within code sequence signal 216 of downlink signal 212.Central terminal 10 is considered to have a master code sequence andsubscriber terminal 20 is considered to have a slave code sequence.Receiver 202 incrementally adjusts the phase of its slave code sequenceto recognize a match to master code sequence and place receiver 202 ofsubscriber terminal 20 in phase with transmitter 200 of central terminal10. The slave code sequence of receiver 202 is not initiallysynchronized to the master code sequence of transmitter 200 and centralterminal 10 due to the path delay between central terminal 10 andsubscriber terminal 20. This path delay is caused by the geographicalseparation between subscriber terminal 20 and central terminal 10 andother environmental and technical factors affecting wirelesstransmission.

FIG. 11 illustrates how receiver 202 of subscriber terminal 20 adjustsits slave code sequence to match the master code sequence of transmitter200 in central terminal 10. Receiver 202 increments the phase of theslave code sequence throughout the entire length of the master codesequence within downlink signal 212 and determines a signal qualityestimate by performing a power measurement on the combined power of theslave code sequence and the master code sequence for each incrementalchange in the phase of the slave code sequence. The length of the mastercode sequence is approximately 100 microseconds based on a chip periodof 2.56 MegaHertz. The phase of the slave code sequence is adjusted byone half of a chip period for each incremental interval during theacquisition phase. Receiver 202 completes a first acquisition pass whenit identifies a correlation peak where the combined power reaches amaximum value. Receiver 202 performs a second acquisition passthroughout the entire length of the code sequence to verifyidentification of the maximum value of the combined power at thecorrelation peak. The approximate path delay between subscriber terminal20 and central terminal 10 is determined when the correlation peakposition is identified in the acquisition mode.

Once acquisition of downlink signal 212 is achieved at receiver 202,fine adjustments are made to the phase of the slave code sequence inorder to maintain the phase matching of the slave code sequence with themaster code sequence in the tracking mode. Fine adjustments are madethrough one sixteenth of a chip period incremental changes to the phaseof the slave code sequence. Fine adjustments may be performed in eitherforward (positive) or backward (negative) directions in response to thecombined power measurements made by receiver 202. Receiver 202continuously monitors the master code sequence to ensure that subscriberterminal 20 is synchronized to central terminal 10 for the downlinkcommunication path.

FIG. 12 shows a graph of the combined power curve measured by receiver202 during the acquisition mode and the tracking mode. The maximum valueof the combined power occurs at the correlation peak 219 of the combinedpower curve. It should be noted that the peak 219 may not be as welldefined as in FIG. 12, but may be flattened at the top, more in the formof a plateau. This is the point where the slave code sequence ofreceiver 202 is in phase with and matches the master code sequence oftransmitter 200. Measurements resulting in combined power values thatoccur off of correlation peak 219 require incremental adjustments to bemade to the slave code sequence. A fine adjustment window is establishedbetween an early correlator point 221 and a late correlator point 223.An average power measurement is performed at early correlator point 221and at late correlator point 223. Since early correlator point 221 andlate correlator point 223 are spaced one chip period apart, an errorsignal is produced upon calculating the difference between the averagepowers of early correlator point 221 and late correlator point 223 thatis used to control the fine adjustments to the phase of the slave codesequence.

After acquiring and initiating tracking on the central terminal 10master code sequence of code sequence signal 216 within downlink signal212, receiver 202 enters a frame alignment mode in order to establishthe downlink communication path. Receiver 202 analyzes frame informationwithin frame information signal 218 of downlink signal 212 to identify abeginning of frame position for downlink signal 212. Since receiver 202does not know at what point in the data stream of downlink signal 212 ithas received information, receiver 202 must search for the beginning offrame position in order to be able to process information received fromtransmitter 200 of central terminal 10. Once receiver 202 has identifiedone further beginning of frame position, the downlink communication pathhas been established from transmitter 200 of central terminal 10 toreceiver 202 of subscriber terminal 20.

FIG. 13 shows the general contents of frame information signal 218.Frame information signal 218 includes an overhead channel 224, a firstuser channel 226, a second user channel 228, and a signaling channel 230for each frame of information transported over downlink signal 212.Overhead channel 224 carries control information used to establish andmaintain the downlink and uplink communication paths. First user channel226 is used to transfer traffic information to first user 208. Seconduser channel 228 is used to transfer traffic information to second user210. Signaling channel 230 provides the signaling information tosupervise operation of subscriber terminal 20 telephony functions.Overhead channel 224 occupies 16 kilobits per second of a frame ofinformation, first user channel 226 occupies 64 kilobits per second of aframe of information, second user channel 228 occupies 64 kilobits persecond of a frame of information, and signaling channel 230 occupies 16kilobits per second of a frame of information.

FIG. 14 shows how overhead channel 224 is inserted into the data streamof downlink signal 212. The data stream of downlink signal 212 ispartitioned into twenty bit subframes. Each twenty bit subframe has twoten bit sections. A first ten bit section includes an overhead bit, asignaling bit, and eight first user bits. A second ten bit sectionincludes an overhead bit, a signaling bit, and eight second user bits.This twenty bit subframe format is repeated throughout an entire fourmillisecond frame of information. Thus, an overhead bit occupies everytenth bit position of frame information in the data stream of downlinksignal 212.

Overhead channel 224 includes eight byte fields--a frame alignment word232, a code synchronization signal 234, a power control signal 236, anoperations and maintenance channel signal 238, and four reserved bytefields 242. Frame alignment word 232 identifies the beginning of frameposition for its corresponding frame of information. Codesynchronization signal 234 provides information to controlsynchronization of transmitter 204 in subscriber terminal 20 to receiver206 in central terminal 10. Power control signal 236 providesinformation to control transmitting power of transmitter 204 insubscriber terminal 20. Operations and maintenance channel signal 238provides status information with respect to the downlink and uplinkcommunication paths and a path from the central terminal to thesubscriber terminal on which the communication protocol which operateson the modem shelf between the shelf controller and the modem cards alsoextends.

In order to identify two successive beginning of frame positions,receiver 202 of subscriber terminal 20 searches through the ten possiblebit positions in the data stream of downlink signal 212 for overheadchannel 224 and frame alignment word 232. Receiver 202 initiallyextracts a first bit position of every ten bit section of frameinformation to determine if overhead channel 224 has been captured. Ifframe alignment word 232 has not been identified after a predeterminedperiod of time from the extraction of the first bit position, receiver202 will repeat this procedure for the second bit position of each tenbit section and subsequent bit positions until frame alignment word 232has been identified. An example of a frame alignment word 232 whichreceiver 202 would search for is binary 00010111. Once the correct bitposition yields frame alignment word 232, receiver 202 attempts toidentify two successive beginning of frame positions. A downlinkcommunication path is established upon the successful identification oftwo successive beginning of frame positions in response to recognitionof successive frame alignment words 232 in the data stream of downlinksignal 212.

Receiver 202 continues to monitor the appropriate bit position in orderto recognize subsequent frame alignment words 232 for subsequent framesof information. If receiver 202 fails to recognize a frame alignmentword 232 for three successive frames, then receiver 202 will return tothe search process and cycle through each of the bit positions of theten bit section until identifying two successive beginning of framepositions through recognition of two successive frame alignment words232 and reestablishing frame alignment. Failure to recognize threesuccessive frame alignment words 232 may result from a change in thepath delay between central terminal 10 and subscriber terminal 20.Receiver 202 will also return to the search process upon an interruptionin the downlink communication path from transmitter 200 in centralterminal 10 to receiver 202 in subscriber terminal 20.

Upon establishing the downlink communication path from central terminal10 to subscriber terminal 20 through proper code sequence phasesynchronization and frame alignment, wireless telecommunication system 1performs procedures to establish the uplink communication path fromtransmitter 204 in subscriber terminal 20 to receiver 206 in centralterminal 10. Initially, transmitter 204 is powered off until thedownlink communication path has been established to prevent transmitterinterference of central terminal communications with other subscriberterminals. After the downlink communication path is established,transmitting power of transmitter 204 is set to a minimum value oncommand from the central terminal CT via power control channel 236 ofoverhead channel 224. Power control signal 236 controls the amount oftransmitting power produced by transmitter 204 such that centralterminal 10 receives approximately the same level of transmitting powerfrom each subscriber terminal 20 serviced by central terminal 10.

Power control signal 236 is transmitted by transmitter 200 of centralterminal 10 in overhead channel 224 of frame information signal 218 overdownlink signal 212. Receiver 202 of subscriber terminal 20 receivesdownlink signal 212 and extracts power control signal 236 therefrom.Power control signal 236 is provided to transmitter 204 of subscriberterminal 20 and incrementally adjusts the transmitting power oftransmitter 204. Central terminal 10 continues to incrementally adjustthe transmitting power of transmitter 204 until the transmitting powerfalls within a desired threshold range as determined by receiver 206.Adjustments to the transmitting power initially occur in a coarseadjustment mode having one decibel increments until the transmittingpower falls within the desired threshold range. Upon turning transmitter204 on, the transmitting power is gradually ramped up in intensitythrough incremental adjustments in order to avoid interference ofcentral terminal communications with other subscriber terminals.

FIG. 15 shows an example decoding scheme for power control signal 236.After the transmitting power of transmitter 204 in subscriber terminal20 reaches the desired threshold range, receiver 206 in central terminal10 continues to monitor the amount of transmitting power fromtransmitter 204 for any changes resulting from power fluctuations, andvariations in the path delay between central terminal 10 and subscriberterminal 20, et al. If the transmitting power falls below or exceeds thedesired threshold range, central terminal 10 will send an appropriatepower control signal 236 to increase or decrease the transmitting powerof transmitter 204 as needed. At this point, adjustments made to returnthe transmitting power to the desired threshold range may occur in afine adjustment mode having 0.1 decibel increments. Upon an interruptionin the downlink or uplink communication paths, central terminal 10 maycommand transmitter 204 to return to a previous transmitting power levelthrough recovery of parameters stored in a memory in subscriber terminal20 in order to facilitate reestablishment of the appropriatecommunication path.

To fully establish the uplink communication path from subscriberterminal 20 to central terminal 10, transmitter 204 in subscriberterminal 20 should be synchronized to receiver 206 in central terminal10. Central terminal 10 controls the synchronization of transmitter 204through code synchronization signal 234 in overhead channel 224 of frameinformation signal 218. Code synchronization signal 234 incrementallyadjusts a phase of the slave code sequence of transmitter 204 to matchthe phase of the master code sequence of receiver 206. Synchronizationof transmitter 204 is performed in a substantially similar manner assynchronization of receiver 202.

Code synchronization signal 234 is transmitted by transmitter 200 incentral terminal 10 in overhead channel 224 of frame information signal218 over downlink signal 212. Receiver 202 of subscriber terminal 20receives downlink signal 212 and extracts code synchronization signal234 therefrom. Code synchronization signal 234 is provided totransmitter 204 for incrementally adjustment of the phase of the slavecode sequence of transmitter 204. Central terminal 10 continues toincrementally adjust the phase of the slave code sequence of transmitter204 until receiver 206 recognizes a code and phase match between theslave code sequence of transmitter 204 and the master code sequence ofcentral terminal 10.

Receiver 206 performs the same power measurement technique indetermining a phase and code match for transmitter 204 synchronizationas performed for receiver 202 synchronization. Adjustments to the phaseof the slave code sequence of transmitter 204 initially occur in acoarse adjustment mode having one half of a chip rate increments untilreceiver 206 identifies the maximum power position of the combined powerof the master code sequence and the slave code sequence of transmitter204.

FIG. 16 shows an example decoding scheme for code synchronization signal234. After identification and verification of a phase and code match ofthe slave code sequence to the master code sequence, receiver 206continues to monitor uplink signal 214 for changes in the phase of theslave code sequence of transmitter 204 resulting from variations in thepath delay between central terminal 10 and subscriber terminal 20. Iffurther adjustments are needed to the phase of the slave code sequenceof transmitter 204, central terminal 10 will send appropriate codesynchronization signals 234 to increase or decrease the phase of theslave code sequence of transmitter 204 as needed. At this point,adjustments made to the phase of the slave code sequence of transmitter204 may occur in a fine adjustment mode having one sixteenth of a chiprate increments. Upon an interruption in the downlink or uplinkcommunication paths, central terminal 10 may command transmitter 204 toreturn to a previous slave code sequence phase value through recovery ofparameters stored in a memory in subscriber terminal 20 in order tofacilitate reestablishment of the appropriate communication path.

After synchronization of transmitter 204 is achieved, receiver 206performs frame alignment on uplink signal 214 in a similar manner asframe alignment is performed by receiver 202 during establishment of thedownlink communication path. Once receiver 206 recognizes two successiveframe alignment words and obtains frame alignment, the uplinkcommunication path has been established. Upon establishing both thedownlink and the uplink communication paths, information transferbetween first user 208 or second user 210 of subscriber terminal 20 andusers coupled to central terminal 10 may commence.

Wireless telecommunication system 1 is capable of adjusting thetransmitting power level and the transmit rate to one of two settingsfor each of three different system operating modes. The system operatingmodes are acquisition, standby and traffic. Adjustments in thetransmitting power and the transmit rate make it possible to reduce andminimize interference with other subscriber terminals. Improvements inlink establishment time are also achieved. The transmitting power levelis decoded in power control signal 236 and the transmit rate is decodedin code synchronization signal 234.

The transmitting power for both downlink signal 212 and uplink signal214 can be set to either a nominal 0 decibel high power level or areduced -12 decibel low power level. The transmit rate for both downlinksignal 212 and uplink signal 214 can be set to a low rate of 10 kilobitsper second or a high rate of 160 kilobits per second. When switched tothe high rate of 160 kilobits per second, user traffic and overheadinformation are spread such that one information symbol results in thetransmission of 16 chips. Correlation is performed over 16 chips,yielding a processing gain of 12 decibels. When switched to the low rateof 10 kilobits per second, only overhead information is spread such thatone overhead symbol results in the transmission of 256 chips.Correlation is performed over 256 chips, yielding a processing gain of24 decibels.

FIG. 17 show the transmitting power and transmit rate for each of thethree system operating modes. At power up or whenever the downlink oruplink communication paths are lost, wireless telecommunication system 1enters the acquisition mode. During the acquisition mode, thetransmitting power of the downlink and uplink transmitters are maximizedas well as the correlator processing gain. This maximizes the signal tonoise ratio at the correlator output, increasing the amplitude of thecorrelation peak 219 for easier identification and minimal risk of falseacquisition. Since only overhead information is needed in theacquisition mode, the transmit rate is at the low rate level of 10kilobits per second.

When the downlink and the uplink communication paths are acquired,wireless telecommunication system 1 enters the standby mode. In thestandby mode, the transmitting power of the downlink and uplinktransmitters are reduced by 12 decibels. This reduction in transmittingpower minimizes the interference to other subscriber terminals whilestill maintaining synchronization. The transmit rate remains at the lowrate level to allow exchange of control information between centralterminal 10 and subscriber terminal 20 over overhead channel 224.

When either an incoming or outgoing call is detected, a message is sentfrom the originating terminal to the destination terminal indicatingthat the downlink and uplink communication paths are required for thetransmission of user traffic information. At this point, wirelesstelecommunication system 1 enters into the traffic mode. During thetraffic mode, the transmitting power of both the downlink and uplinkcommunication paths is increased to the high power level and thetransmit rate is increased to the high rate level of 160 kilobits persecond to facilitate information transfer between originating anddestination terminals. Upon detection of call termination, a message issent from the terminating terminal to the other terminal indicating thatthe downlink and uplink communication paths are no longer required. Atthis point, wireless telecommunication system 1 reenters the standbymode. Code synchronization and frame alignment tracking is performed inboth the standby mode and the traffic mode.

FIG. 18 is a detailed block diagram of receiver 202 and transmitter 204in subscriber terminal 20. Receiver 202 receives downlink signal 212 atan RF receive interface 250. RF receive interface 250 separates thespread spectrum signal into I and Q signal components. RF receiveinterface 250 band pass filters each of the I and Q signal components byremoving portions above approximately half of receiver 202 bandwidth of3.5 MegaHertz. RF receive interface 250 low pass filters the I and Qsignal components to reject image frequencies and prevent signalaliasing. The I and Q signal components are placed into digital formatby an analog to digital converter 252. The sampling frequency of analogto digital converter 252 is four times the chip period, or 10.24MegaHertz, with an eight bit resolution.

The digital I and Q signal components are stepped to a rate of 5.12MegaHertz by a down converter 254. A code generator and despreader 256performs the synchronization acquisition and tracking functionspreviously described to synchronize the phase of the Rademacher-Walshand pseudorandom noise code sequence of receiver 202 to that of downlinksignal 212. A digital signal processor 258 controls the phase of theslave code sequence through a code tracker 260 and a carrier tracker262. An automatic gain control unit 264 produces an automatic gaincontrol signal to control the gain of RF receive interface 250. Codegenerator and despreader 256 generates the I and Q 160 kilobits persecond of frame information for further synchronization by a node syncinterface 266 under the control of a node sync logic unit 268. Node syncinterface 266, through node sync logic unit 268, determines whether theI and Q channels should be swapped, as they may be received in fourdifferent ways.

Viterbi decoder 270 provides forward error correction on the I and Qchannels and generates an error corrected 160 kilobits per second datasignal after a 71 symbol delay. The error corrected signal is processedby a frame aligner and extractor 272 determines frame alignment andextracts power control signal 236, code synchronization 234, andoperations and maintenance channel signal 238. Frame aligner andextractor 272 also extracts first user channel 226 and second userchannel 228 for traffic transmission towards first user 208 and seconduser 210, and signaling channel 230 for processing by high level datalink controller 274 and a microcontroller 276. Frame aligner andextractor 272 also provides alarm and error indications upon detecting aloss in frame alignment. A non-volatile random access memory 278 storessystem parameter information for subsequent insertion through anarbitrator 280 in the event of link loss in order to facilitate linkreestablishment. Arbitrator 280 also provides an interface betweendigital signal processor 258 and microcontroller 276.

In the transmit direction, a frame inserter 282 receives first usertraffic and second user traffic from first user 208 and second user 210,signaling channel 230 information from high level data link controller274, and operations and maintenance channel 238 information frommicrocontroller 276. Frame inserter generates frame information signal218 for uplink signal 214 for processing by a convolutional encoder 284.Convolutional encoder 284 doubles the data rate of frame informationsignal 218 to provide forward error correction. A spreader 286 splitsthe 320 kilobits per second signal of convolutional encoder 284 into two160 kilobits per second I and Q signals and exclusively ORs thesesignals with the spreading sequence generated by a code generator 288 inresponse to a system clock generated by clock generator 290 as adjustedby code synchronization signal 234. Code generator 288 generates one ofsixteen Rademacher-Walsh functions exclusive ORed with a pseudo-randomsequence having a pattern length of 256 with a chip rate of 2.56MegaHertz. The pseudo-random sequence should match that of centralterminal 10, but is adjustable under software control to providereliable rejection of signals from other bands or other cells.

Spreader 286 supplies the I and Q signals to an analog transmitter 290.Analog transmitter 290 produces pulsed I and Q signals for an RFtransmit interface 292. Transmit power is generated by firstestablishing a control voltage from a digital to analog converter inresponse to power control signal 236 extracted from overhead channel224. This control voltage is applied to the power control inputs ofanalog transmitter 290 and RF transmit interface 292. Power control of35 decibels is obtainable in both analog transmitter 290 and RF transmitinterface 292. RF transmit interface 292 includes a step attenuator thatprovides 2 decibel steps of attenuation over a 30 decibel range. Thisattenuator is used to switch between high and low power levels. On powerup, maximum attenuation is selected to minimize the transmitting powerof transmitter 204.

In summary, a wireless telecommunication system provides wirelesstelephone type communications for users remote from the public telephoneswitching network. The wireless telecommunication system includes acentral terminal that communicates through CDMA spread spectrum radiofrequency transmissions to a plurality of users serviced by subscriberterminals.

Thus, it is apparent that there has been provided, in accordance withthe present invention, an apparatus and method of synchronizing atransmitter in a subscriber terminal of a wireless telecommunicationssystem that satisfies the advantages set forth above. Although thepreferred embodiment has been described in detail, it should beunderstood that various changes, substitutions, and alterations can bemade herein. For example, though the downlink and uplink signals havebeen described with a specific format and rate, other formats and ratesmay be implemented while providing a similar transmission of status,control, and information. Although a particular embodiment has beendescribed herein, it will be appreciated that the invention is notlimited thereto and that many modifications and additions thereto may bemade within the scope of the invention.

What is claimed is:
 1. A method of transmitting information in awireless telecommunications system, comprising the steps of:transmittinga downlink signal at a first transmitting power and a first transmittingrate in an acquisition mode; transmitting the downlink signal at asecond transmitting power and the first transmitting rate in a standbymode upon completion of the acquisition mode, the second transmittingpower being less than the first transmitting power.
 2. The method ofclaim 1, further comprising the step of: transmitting the downlinksignal at the first transmitting power and a second transmitting rate ina traffic mode upon a request for a wireless telephone call.
 3. Themethod of claim 1, wherein the downlink signal includes an overheadchannel, a signalling channel, a first user channel, and a second userchannel for the second transmitting rate.
 4. The method of claim 3,wherein the second transmitting rate is 160 kilobits per second.
 5. Themethod of claim 1, wherein the downlink signal includes an overheadchannel for the first transmitting rate.
 6. The method of claim 5,wherein the first transmitting rate is 10 kilobits per second.
 7. Thesystem of claim 6, wherein the transmitter transmits the downlink signalat the first transmitting power and a second transmitting rate upon arequest for a wireless telephone call, the receiver operable to receivethe downlink signal at the second transmitting rate.
 8. The system ofclaim 7, wherein the transmitter transmitts the downlink signal in thestandby mode at the second transmitting power and the the firsttransmitting rate in response to a sompletion of the wireless telephonecall, the receiver operable to receive the downlink signal at the firsttransmitting rate.
 9. The system of claim 8, wherein the firsttransmitting rate is at 10 kilobits per second, the downlink signalincluding only an overhead channel for the first transmitting rate. 10.The system of claim 7, wherein the second transmitting rate is at 160kilobits per second, the downlink signal including an overhead channel,a signalling channel, a first user channel, and a second user channelfor the second transmitting rate.
 11. The system of claim 7, wherein thereceiver simultaneously changes its operating rate with a change in thetransmitting rate of the downlink signal at the transmitter.
 12. Thesystem of claim 9, wherein the second transmitting rate is at 10kilobits per second, the downlink signal including an overhead channel,a signaling channel, a first user channel, and a second user channel forthe second transmitting rate.
 13. The subscriber terminal of claim 12,wherein the downlink signal has the second transmitting power and thefirst transmitting rate of the standby mode in response to a completionof the wireless communication transmission, the receiver adjusting tooperate at the first transmitting rate in the standby mode.
 14. Thesubscriber terminal of claim 12, wherein the receiver processes thedownlink channel that includes an overhead channel, a signallingchannel, a first user channel, and a second user channel for the secondtransmitting rate.
 15. The subscriber terminal of claim 14, wherein thesecond transmitting rate of the downlink signal is 160 kilobits persecond.
 16. The subscriber terminal of claim 14, wherein the receiverprocesses the downlink signal that includes only the overhead channelfor the first transmitting rate.
 17. The subscriber terminal of claim19, wherein the first transmitting rate of the downlink signal is 10kilobits per second.